1. Field of the Invention
The present invention relates to the field of digital frequency synthesizers and in particular, relates to musical tone generators and frequency synthesizers.
2. Prior Art
Frequency synthesizers are categorized as either analog or digital. In each category generators have been devised to produce one frequency at a time or multiple frequencies simultaneously.
Analog frequency synthesizers have been generally characterized by requiring a distinct electrical component for each discrete frequency. In other words, to create a number of frequencies an equal number of components such as resistors, inductors, switching circuitry or oscillators are required to simultaneously create the same number of frequencies. Complex switching circuits are devised to control switching between a smaller number of controlling components and a larger number of controlled oscillators, or tone generators. Large and complex circuits are the result.
The design or digital circuitry often parallels prior analog circuitry used for simultaneous generation of frequencies. Such digital circuits also incorporate a generally linear increase in component count with an increase in the number of frequencies generated. For example, a separate oscillator is required for each frequency, such as a flip-flop, phase-locked-loop or monostable oscillator. Electronic or mechanical switching between frequency determining components such as resistors or crystals is also used in order to control as many oscillators as frequencies which are required. A small set of fixed frequencies may be heterodyned to create a larger set of frequencies. In the heterodyning method, switching complexity increases as the number of simultaneous frequencies also increases. In addition, when heterodyning the set of fixed frequencies necessarily becomes even larger when the frequencies which are ultimately desired are not simply related. When digital counters are used as the basic element in frequency generators, the result is that the number of digital counters required equals the number of desired frequencies. The prior art uses a small number of separate oscillators to clock a number of counters to provide in turn a multiplicity of low frequency signals. Each desired frequency thus requires a separate counter. Shift registers have been used in the same manner as counters to produce a multiplicity of low frequency signals.
What is needed then is circuitry and a methodology for simultaneously producing a large number of frequencies without necessitating a corresponding increase in the number of separable elements required to generate the number of discrete frequencies desired.